1. Field of the Invention
The invention relates to a running road determination device and a vehicle running control apparatus and control method equipped with the running road determination device, and more particularly, to a running road determination device for determining whether or not a vehicle is running on a road difficult to run on, and a vehicle running control apparatus equipped with the running road determination device.
2. Description of the Related Art
A vehicle is mounted with a vehicle running control apparatus that performs automatic running control, such as constant-speed running control as constant vehicle speed control to make the vehicle speed of the vehicle equal to a target vehicle speed, follow-up running control as vehicle speed control to cause the own vehicle to run while following a preceding vehicle, namely, adaptive cruise control (ACC), and the like. In the vehicle running control apparatus, an engine for applying a driving torque to the vehicle and a brake device for applying a braking torque to the vehicle are cooperatively controlled such that the vehicle speed becomes equal to the target vehicle speed. In the vehicle running control apparatus, an automatic running control ECU calculates a target driving torque such that the vehicle speed of the vehicle becomes equal to the target vehicle speed, the calculated target driving torque is output to an engine ECU; and the engine ECU controls the engine as a braking/driving torque generating device on the basis of the target driving torque. Further, in the vehicle running control apparatus, the automatic running control ECU calculates a target braking torque as a target control torque such that the vehicle speed of the vehicle becomes equal to the target vehicle speed, the calculated target braking torque is output to a brake ECU, and the brake ECU controls the brake device as a braking/driving torque generating device on the basis of the target braking torque.
In recent years, some vehicle running control apparatuses perform automatic running control at a low target speed, for example, about 10 km/h. For example, as disclosed in Japanese Patent Application Publication No. 2004-90679 (JP-A-2004-90679), there is a vehicle running control apparatus that performs automatic running control at a target speed approximately equal to a creep speed.
It should be noted herein that roads on which a vehicle runs are classified into on-roads and off-roads. The off-roads are classified into roads relatively easy for the vehicle to run on and roads relatively difficult for the vehicle to run on, depending on the conditions of the roads. Even in the case where automatic running control is performed on a load difficult for the vehicle to run on, the vehicle running control apparatus controls the braking/driving torque generating devices to maintain the target vehicle speed. For example, in the case where the road is composed of rocks insurmountable by tires fitted to the vehicle, the case where the road is a muddy road with a low coefficient of friction, the case where the vehicle is in contact with an obstacle, or the like, the vehicle has difficulty in moving forward. In such a case, when the braking/driving torque generating devices are controlled to increase the driving torque applied to the vehicle, there is a problem in that an excessive load is applied to the braking/driving torque generating devices or the vehicle. It is therefore desirable to reliably determine whether or not the vehicle is running on a road difficult to run on.
The invention provides a running road determination device capable of reliably making a determination on a road on which a vehicle is running, and a vehicle running control apparatus capable of restraining an excessive load from being applied to braking/driving torque generating devices or a vehicle.